José Mourinho has said that it hurts him to leave Juan Mata out of his starting 11 but the Chelsea manager insists that he has to have a "hard heart" when making difficult decisions about his team and reiterated that the Spaniard will not be leaving Stamford Bridge in January.

Despite winning Chelsea's player of the year award for the past two seasons, Mata has found his opportunities restricted since Mourinho's return to Stamford Bridge last summer. The midfielder was furious to be substituted shortly after half-time during the 3-0 win at Southampton on New Year's Day, raising speculation that he might be sold this month.

Mata's agents subsequently sought clarification with the club over his future, during which they were reassured that he is not for sale. "If people think they can get our good players on sale, sales finished in London yesterday," Mourinho said. "I keep saying that I want him to stay. There [are no] offers. We have no offers."

Mourinho, who denied reports that Chelsea have had a £50m bid rejected for Gonzalo Higuaín by Napoli, said that Mata has trained "fantastically" for Sunday's trip to Derby County in the third round of the FA Cup and hopes he can still be a key player. "I try to do my job forgetting I'm a soft heart, pretending that I'm a hard heart," he said. "Many times it hurts me, not just with him, with decisions I have to make. But I think to do my job in the best way … thinking the team more important than any player. When I have to make a decision I always try to be a cold one, icy, analyse the situation and try to make the best decision for the team … putting the person secondary.

"But yes, it hurts me. He's a good kid, he works hard, not an easy situation for him not to be playing every time. He started nine matches in the Premier League, had lots of minutes on the pitch. I love it if he does well. That's what I want."

Mourinho was adamant that Chelsea will not test Napoli's resolve with an offer for Higuaín this month, even though he knows the Argentina striker from his time at Real Madrid. While a new striker is not a priority, Chelsea do hope to bring the Internazionale midfielder Fredy Guarín, to England this month. Chelsea are prepared to pay £12.4m for the Colombian but Inter are holding out for £16.6m.

That move may hinge on whether Kevin De Bruyne leaves Chelsea. The unsettled Belgian midfielder has been linked with Bayer Leverkusen, Wolsburg and Málaga and his agent has said that he favours a move to Germany.

However Chelsea will not let De Bruyne leave on the cheap and are thought to want £25m for him.

Mourinho, who will play a strong side against Derby, reacted strongly to the suggestion by the Aston Villa manager Paul Lambert that the FA Cup is no longer an important competition and said that some youngsters are more interested in earning money than winning trophies. He believes that they are influenced by people around them and said he had rejected players for that reason.

"In the past players made money in their career," Mourinho said. "Now they want money before their career. They're surrounded by people who want to make money before career starts. Society has changed, football has changed, people around players changed and players changed.

"I'm referring more to players at the beginning of their career. I don't know a top player who only thinks money. All the top players have all the ingredients but people at beginning of careers, instead of being ready to destroy everything in front of them to reach a dream, look that money is more important before playing one game for the first team."Twenty years ago they would pay for that dream. I would have at 15 or 16, I would have paid to play one game."